Ladder-type filters are used in high frequency communication systems. It has been known to connect an inductor between a parallel resonator of a ladder-type filter and ground as disclosed in Japanese Patent Application Publication Nos. 2012-231437 and 2002-300003 and International Publication No. 2007-015331. It has been also known to unify and connect ground terminals of parallel resonators to ground as disclosed in Japanese Patent Application Publication Nos. 2012-506185, 2008-271511, and 2003-069382.
The connection of an inductor between a parallel resonator and ground improves attenuation characteristics. As the number of inductors is increased, the attenuation characteristics are improved more. However, multiple large inductors make it difficult to reduce the size of the ladder-type filter.